1. Field of the Invention
The field of the invention is magnetic resonance imaging (MRI) and, in particular, local coils for use in receiving MRI signals.
2. Background Art
A. Magnetic Resonance Imaging
In MRI, a uniform magnetic field B.sub.0 is applied to an imaged object along the z-axis of a Cartesian coordinate system the origin of which is approximately centered within the imaged object. The effect of the magnetic field B.sub.0 is to align the object's nuclear spins along the z-axis.
In response to a radio frequency (RF) excitation signal of the proper frequency, oriented within the x-y plane, the nuclei precess about the z-axis at their Larmor frequencies according to the following equation: EQU .omega.=.gamma.B.sub.0 ( 1)
where .omega. is the Larmor frequency, and .gamma. is the gyromagnetic ratio which is constant and a property of the particular nuclei.
Water, because of its relative abundance in biological tissue and the properties of its nuclei, is of principle concern in such imaging. The value of the gyromagnetic ratio .gamma. for water is 4.26 kHz/gauss and therefore, in a 1.5 Tesla polarizing magnetic field B.sub.0, the resonant or Larmor frequency of water is approximately 63.9 MHz.
In a typical imaging sequence for an axial slice, the RF excitation signal is centered at the Larmor frequency .omega. and applied to the imaged object at the same time as a magnetic field gradient G.sub.z is applied. The gradient field G.sub.z causes only the nuclei in a slice through the object along an x-y plane, to have the resonant frequency .omega. and to be excited into resonance.
After the excitation of the nuclei in this slice, magnetic field gradients are applied along the x and y axes. The gradient along the x axis, G.sub.x, causes the nuclei to precess at different frequencies depending on their position along the x axis, that is, G.sub.x spatially encodes the precessing nuclei by frequency. The y axis gradient, G.sub.y, is incremented through a series of values and encodes the y position into the rate of change of phase of the precessing nuclei as a function of gradient amplitude, a process typically referred to as phase encoding.
A weak nuclear magnetic resonance generated by the precessing nuclei may be sensed by the RF coil and recorded as an NMR signal. From this NMR signal, a slice image may be derived according to well known reconstruction techniques. An overview of NMR image reconstruction is contained in the book "Magnetic Resonance Imaging, Principles and Applications" by D.N. Kean and M.A. Smith.
B. Local Coils
The quality of the image produced by MRI techniques is dependent, in part, on the strength of the NMR signal received from the precessing nuclei. For this reason, it is known to use an independent RF receiving coil placed in close proximity to the region of interest of the imaged object to improve the strength of this received signal. Such coils are termed "local coils" or "surface coils". The smaller area of the local coil permits it to accurately focus on NMR signals from the region of interest. Further, the RF energy received by a local coil is obtained from a smaller volume giving rise to improved signal-to-noise ratio in the acquired NMR signal.
The improved signal-to-noise ratio of a local coil comes at the cost of a reduced field-of-view, that is, a reduced volume of the patient over which the coil is sufficiently sensitive to detect the NMR signal. This reduced field-of-view is a direct result of the smaller coil size of the local coil.
It is known to use multiple local coils to increase the field-of-view. In order to maintain a high signal-to-noise ratio in such multiple local coil constructions, it is critical that the electrical interaction between local coils be minimized--otherwise the multiple, local coils devolve to a single larger coil without the signal-to-noise advantages of the individual local coils.
U.S. Pat. No. 4,825,162, issued Apr. 25, 1989, (the "Roemer" patent) assigned to the same assignee as the present invention, and hereby incorporated by reference, describes a local coil array in which a plurality of single turn coils are arrayed over a surface (not necessarily a plane) so as to provide a large field-of-view and the signal-to-noise benefits of the individual local coils. Isolation between the loops of the coil array is maintained by adjusting the degree of overlap between the loops so that they are electromagnetically decoupled and by connecting each loop to an extremely low impedance preamplifier.
As described in the Roemer patent, the low impedance preamplifiers used with each coil effectively reduce the interaction of the individual coils by a factor of Q/N where Q is the quality factor of each coil and N is the ratio between the output capacitance of each coil and its net series capacitance.
Although the Roemer patent contemplates that its coil array may follow a three dimensional object by a "wrapping" of the array around that object, for small diameter objects, such as knees or portions of other extremities, this approach is not completely successful. If the radius of curvature of the wrapping is sufficiently small, the isolation between the individual coils breaks down, importantly, non-adjacent coils along the surface of the array may interact when they become opposed across the extremity. This interaction of opposed coils is stronger than that which may be nullified by the preamplifier decoupling mechanism described in the Roemer patent.